The present invention has its main application within the field of sensors and relates to a method for association between tracks of moving measurement objects, in particular when a pause has occurred between the target tracks between which association is to be performed.
Certain types of sensor, for example search radar systems or surveillance radar systems, do not measure the position of one and the same measurement object constantly but instead measure at discrete times the positions of a plurality of measurement objects within a given area or volume. In connection with this type of sensor, it may be desirable to obtain information about where a measurement object will be located at a point in time before the next measurement time, in other words it is desirable to be able to predict the position of the measurement object, the target, before the next measurement time.
Such prediction is often referred to as target tracking and is performed on the basis of a number of consecutive measurement times which are hereinafter referred to as detections. Target tracking provides information about, for example, the speed, course and acceleration of the target. This information is obtained with the aid of calculations which are performed on the basis of a number of consecutive detections of the target concerned. A problem may arise in the calculations if more than one target is located within the area or volume scanned by the sensor. The problem lies in the uncertainty about which new detections are to be considered to belong to which existing target tracks.
This problem can be solved by so-called association, which is often performed in two stages, correlation and allocation. Correlation usually consists of a method for determining the degree of probability that a new detection belongs to an existing target track. On the basis of the result of the correlation, the new detection can then be allocated to a given existing target track.
A conventional method for association is to predict at the correlation stage where the targets that are being tracked will be located the next time they are detected, after which a tracked target is allocated the new detection that lies closest to the predicted position of the tracked target.
There are a great many methods for obtaining improved reliability in association on the basis of this fundamental principle. One way is, with reference to the last detected position of a tracked target, to create an imaginary area or volume within which it is considered that it will be possible for the tracked target to be located at the next detection. A new detection which is carried out within this area or volume is allocated to the tracked target. Another method is, on the basis of a tracked target, to initiate a number of alternative tracks, so-called hypothetical tracks, in order at a later stage to investigate which of the hypothetical tracks corresponds most closely to a given new detection.
The problem with association between new detections and existing target tracks is rendered even more difficult if the sensor used has made a pause in its scanning of a given area or volume. It has not been possible for the target tracks that were located within this area or volume during the pause to be updated with new detections during the pause. The method described above for calculating possible positions for targets after the pause then leads to unreasonably large areas for the possible positions. In the same way, the method using hypothetical tracks leads to altogether too many allocation possibilities.
European Patent Application EP 607 015 describes a method for association between a new detection and an existing target track with the aid of the first of the methods described above, in other words information about the previous movement of the target is used to predict a volume within which the new position of the target may be.
European Patent Application EP 660 134 describes a method for tracking a number of targets simultaneously. The method includes the formation of hypothetical tracks, in principle according to the second of the two methods described above.